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Planetary News: Stardust (2009)Stardust Retrieves Life Building-Block from CometBy Amir AlexanderAugust 25, 2009
Scientists have identified a crucial building block of life in samples captured from a comet and returned to Earth by the spacecraft Stardust. The discovery lends support to the theory that key ingredients of life arrived on the early Earth from space, through a heavy bombardment of comets and asteroids. The samples were captured by Stardust on January 2, 2004, as the spacecraft passed by comet Wild-2, coming within 240 kilometers of its nucleus. For several hours as it flew through Wild-2's coma, Stardust was subjected to a barrage of rocks and grains from the comet, which battered but did not damage the spacecraft. Along the way, particles from the bombardment became embedded in the spacecraft's collector, made of aerogel tiles framed in aluminum foil. Once the ordeal was over, the collector was tucked into Stardust's sample return capsule and sealed inside. Two years later, on January 15 2006 Stardust swooped by the Earth, and released the capsule into the atmosphere, where it parachuted gently to the ground in the Utah desert. The contents of this capsule, the first ever samples from a comet collected in space and brought to Earth, have been occupying scientists around the world ever since. One remarkable discovery in the Stardust samples took place when Daniel Glavin and Jason Dworkin of NASA's Goddard Space Flight center found traces of glycine embedded in the collector's aluminum foil. Glycine is one of the amino acids that combine to form proteins – the building blocks of living organisms. The discovery of an amino acid in a comet would have profound implications as to the origins and prevalence of life, but scientists were cautious: it was possible, some argued, that the glycine came not from the comet but from contamination of the aluminum on Earth. Amino acids, after all are very common on our own planet. Fortunately, however, there is a way to distinguish Earthly glycine from the same compound originating elsewhere. The method involves analyzing the ratio of isotopes – elements with identical chemical properties but different atomic weights. Carbon, for example, comes in at least two forms: the most common isotope Carbon 12, which has 6 protons and 6 neutrons in its nucleus, and Carbon 13 which is heavier because it has an extra neutron in its nucleus. In any carbon sample taken on earth the ratio between these isotopes is fixed. Different isotopic ratios can only be found in samples that came directly from space, such as in meteorites.
Like other amino acids glycine contains carbon, and this, according to Dr. Jamie Elsila of NASA Goddard was the key for determining whether the samples found in the Stardust collector truly came from space. If the isotopic ratios of were the same as elsewhere on Earth, then the glycine was most likely the result of contamination. But if the isotopic ratios were different, then the glycine almost certainly came from space, in this case from the comet Wild-2. Measuring the relative amounts of Carbon 12 and Carbon 13 in the Stardust glycine samples proved to be a daunting task. The samples were so tiny, Elsila found, that traditional measurement methods were simply not accurate enough. "We spent two years testing and developing our equipment to make it accurate and sensitive enough to analyze such incredibly tiny samples" she said. Finally, however, the results were in: the ratio of the two carbon isotopes in the glycine from Stardust differed by 2.9% from the ratios that prevail on Earth. This difference was sufficient for Elsila and her collaborators to conclude with a high degree of confidence that the glycine found in the collector came from comet Wild-2. "Glycine is an amino acid used by living organisms to make proteins" said Elsila, "and this is the first time an amino acid has been found in a comet." The implications of finding an amino acid on Wild-2 are far reaching. For one, it provides an important clue to the persistent question of just how prevalent life is in the universe. Ancient comets like Wild-2 are essentially balls of ice and debris left over from the formation of the solar system four and a half billion years ago. If even such primitive bodies contain amino acids it suggests that these building blocks of protein are most likely very commonplace, which may imply that life itself is also be quite common. "The discovery of glycine in a comet . . . strengthens the argument that life in the universe may be common rather than rare" commented Carl Pilcher, Director of NASA's Astrobiology Institute.
For another, the discovery of glycene in the Stardust samples also provides important clues as to the origins of life on Earth. According to current theories of planetary formation, the young Earth was poor in elements and compounds known as "volatiles" which are essential for life. Carbon, water, amino acids, and other volatiles evaporated in the high temperatures that prevailed in the early years of the inner solar system, but were preserved in the colder conditions that prevailed further out. Many scientists believe that these ingredients were then brought back to the Earth by an intense bombardment of comets and meteorites that took place about 3.9 billion years ago. "The early Earth was bombarded with comets and meteorites" said Elsila. This is one more clue to what ingredients could have been present on the early Earth and how they could have gotten there. In addition to collecting particles from a comet, Stardust had another mission on its seven-year odyssey: gathering grains of interstellar dust that stream towards the Sun from far beyond our solar system. These grains were also stored in the aerogel collector, but on the opposite side from the grains collected from Wild-2. These interstellar particles are much smaller, fewer, and harder to locate in the collector than the cometary particles. To find them, Andrew Westphal and several collaborators at U.C. Berkeley have launched Stardust@home, a distributed computing project in which volunteers look for the grains on close-up movies of the collector. Working closely with The Planetary Society, Westphal and his team have recruited tens of thousands "Dusters" who are committed to the search for the elusive particles. Once detected, these emissaries from faraway worlds will help scientists decipher the history and evolution of the universe itself. Did you like this story? Send
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